There are many prior art material handling vehicles of the fork lift type which have been designed for various specific needs or uses. Of these prior art types, the more conventional and most widely used require a counterweight disposed near the rear position to provide stability during the lifting and carrying of a load. Some fork lifts provide adjustable counterbalance weights, adjustable according to the weight on the prongs of the fork. While these types of vehicles provide a greater flexibility for given application, they also represent a very expensive investment. In certain agricultural applications which require movement over fields and the like, the great weight of the vehicle is also a disadvantage since they tend to create problems when soil conditions are soft. Thus, there is a need for a light weight vehicle and specifically one without counterweights.
One type of vehicle has attempted to overcome the need for counterweights does so by disposing the fork prongs between a pair of rails or frame members to establish the center of gravity between the rear and front wheels. However, while this configuration reduces the cost and weight, in some instances, it also hinders the usefulness of the vehicle in certain specialized applications. This type of fork lift vehicle is limited to applications wherein the load or pallet fits between the confines of the frame and wherein the pallet is loaded or unloaded only at ground level.
These prior types of vehicles are represented by the U.S. Pat. Nos. 3,861,535; 3,039,647; 3,321,109 and 3,610,453.
Prior to the present invention, there has not been a fork lift vehicle which did not require the conventional counterweights used to stabilize the load and which also permitted the flexibility of loading or unloading at both ground level as well as at an elevated position.
A U.S. Pat. No. 2,667,885 discloses the concept of picking up heavy loads from the ground level and transporting them while the center of gravity of the load is intermediate to the front and rear wheels.
Stabilizing feet affixed to the forward end of a fork lift truck are not new and examples are in U.S. Pat. Nos. 3,235,105 and 3,586,183. The purpose of retractable stabilizing feet on the forward end of a fork lift truck is to provide stability when the lifting is taking place as an alternative to counterweights on the back of fork lift trucks.
This invention uses a chain-link drive mechanism in combination with sprocket wheels to give a positive power drive to the movement of the forks for lifting and lowering loads as needed. Chains are substituted for rigid piston and cylinder combinations because the chains allow more play and flexibility in the mast of a fork lift truck. Chains are not new per se and examples of chain use in fork lift trucks is shown in U.S. Pat. Nos. 4,369,861; 4,621,711; 4,531,615; and 4,312,427.
One of the reasons for the unique structure of this invention is to provide a clear line of sight between the operator and the forward position of the fork lift. Other conventional devices for the most part obstruct the vision of the drive by providing a central bar between the two upstanding shafts forming the mast and/or the driving piston will rise and fall in the area between the shafts. This is a safety hazard in industrial environments. The structure of the instant invention will not have these problems, however, it is acknowledged that merely providing a clear line of sight between the shafts of the mast of the fork lift is not new, see for example U.S. Pat. Nos. 4,356,893 and 4,261,438.